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How To User & File Permissions |
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Jul 12, 2008 at 09:38 AM |
Linux User & File Permissions
Impress tutorial is here.
Users and Groups
o Each file has one user and one group
o Stored as numbers
+ 0 – root
+ 1 – 499 reserved for system use
+ 500/1000+ – users (depending on distro)
o ls -n vs. ls -l
User
o “User” is owner of the file
o Creator by default
o Each “User” belongs to one or more “Groups”
o Group of the same name as the User is default
Basic Permissions
o Three basic permissions
o Read – View a file
+ r
o Write – Write to a file
+ w
o Execute – Run a file (Program)
+ x
Basic Permissions
o The 3 basic permissions are applied to:
o User, Group, and Everyone
+ rwx rwx rwx
o Shown as “-” if the permission is not granted
+ rw- rw- r--
o Can also be listed as an octal number:
+ 110→6 110→6 100→4 = 664
Example
o We get this from the ls -l command:
o User is byronh
o byronh can read and write
o Group is byronh
o Members of byronh can read and write
o Everyone else can read
File Type
o First character is file type
o - is a normal file
File Types
o - – Normal file
o d – directory
o l – symbolic link
o s – socket
o p – pipe
o c – character device
o b – block device
Basic Permissions (directories)
o Read and Write the same
o Execute allows file contents to be listed
Advanced Permissions (programs)
o SUID – set user id
o Allow program to run with “User” permissions
o GUID – set group id
o Allow program to run with “Group” permissions
Advanced Permissions (directory)
o GUID – Newly created files have the same group as the directory
o (instead of the user's default group)
o Sticky bit – save text attribute
o Anyone can create new files, but only users or groups can modify or delete
Overview
o 0 – filetype “-”/d/l/p/c/b
o 1 – User read “-”/r
o 2 – User write “-”/w
o 3 – User execute “-”/x/s/S
o 4 – Group read “-”/r
o 5 – Group write “-”/w
o 6 – Group execute
Setting permissions
o chmod [mode] file(s) [-R]
o Changes permissions on file(s)
o -R recurses through subdirectories
Setting Permissions 2
o [mode] as symbolic
o 3 groupings:
+ ugoa – user/group/other/all (defaults to a)
+ - + = – remove/add/set equal to
+ rwxst – read/write/execute/SUID or GUID/sticky
Setting Permissions 3
o [mode] as numeric
o 3 or 4 digits
+ Last 3 are permissions for User, Group, Other
+ First is optional and used for SUID/GUID/sticky
+ Number represents permissions as binary
Useful commands
o chown – change owner/user and group of file
o ls -l – long file listing
o usermod -G group1,group2 user
o Make user a member of group1 and group2
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